otoutosan
おとうとさん
(someone else's) younger brother
Note
[otootosan]…is used to refer to someone else's younger brother. "otooto" is used to refer to one's own younger brother.
Example
A：Bさんの おとうとさんの おなまえは？
B：おとうとの なまえは いちろうです。
A: What's the name of your younger brother, Mr./Ms. B?
B: His name is Ichiro.